3 Oxygenation Strategies

Choosing oxygen support

Oxygen support aims to correct hypoxemia without unnecessary oxygen exposure and to identify when a patient needs ventilatory or airway support. Supplemental oxygen treats low blood oxygen, but does not by itself correct inadequate ventilation, airway obstruction, or the cause of respiratory failure. The guidance here focuses on adults; children and some patients with specific conditions need different targets and protocols.

Select support according to the severity of hypoxemia, the patient's work of breathing, the oxygen concentration needed, and the response to treatment. Flow and delivered concentration are approximate for many devices and vary with fit, breathing pattern, and equipment. Devices are not simply interchangeable steps on a ladder: use the support suited to the patient's physiology and escalate promptly if it is not achieving the intended response.

Device uses and limitations

Each device has particular uses and limitations:

• Nasal cannula: Low-flow oxygen for mild hypoxemia, commonly $1\text{–}6\ \mathrm{L/min}$. It is comfortable and allows eating and speaking, but delivered concentration varies with the patient's breathing.

• Simple face mask: Often used when a cannula is inadequate. Use a flow of at least $5\ \mathrm{L/min}$ to reduce rebreathing of exhaled gas; fit and breathing pattern affect delivered concentration.

• Venturi mask: Provides a more controlled oxygen concentration and is useful when precise titration is important, including for patients at risk of hypercapnic respiratory failure. Use the specified adapter and flow.

• Reservoir mask (non-rebreather): Provides high-concentration oxygen in urgent or severe hypoxemia. Set sufficient flow to keep the reservoir inflated during inspiration and ensure that the oxygen source works. It does not replace ventilation support.

• High-flow nasal oxygen (HFNO/HFNC): Delivers heated, humidified gas at high flow with adjustable oxygen concentration. It can support patients with acute hypoxemic respiratory failure and may be used during intubation attempts in selected patients. Monitor closely for treatment failure.

• Noninvasive ventilation (NIV/NIPPV): A tight-fitting mask supplies positive pressure, with oxygen as needed. It can improve oxygenation and assist ventilation in appropriate patients, but requires monitoring and may be unsuitable if the patient cannot protect the airway or tolerate the mask.

• Bag-mask ventilation: Used for apnea or inadequate breathing. With a reservoir and oxygen, it can deliver a high oxygen concentration; effective ventilation depends on airway positioning, mask seal, and appropriate technique.

In severe or rapidly worsening respiratory distress, call for appropriately skilled help and prepare for advanced airway support.

Targets and monitoring

For most acutely ill adults, a common target is $\mathrm{SpO_2}$ of $94\text{–}98\%$. For adults with COPD or other recognized risk factors for hypercapnic respiratory failure, an initial target of $88\text{–}92\%$ is commonly recommended pending blood-gas assessment and individualized clinical review. Follow local protocols and any documented patient-specific target.

Titrate oxygen to the target rather than routinely aiming for $100\%$; excessive oxygen can be harmful in some patients. Use pulse oximetry alongside clinical assessment, and check that the signal is reliable. Motion, poor peripheral perfusion, and other factors can impair accuracy, so interpret the reading in context.

A normal $\mathrm{SpO_2}$ does not establish that ventilation is adequate or that carbon dioxide is normal. After each change in support, reassess oxygen saturation, respiratory effort, mental status, and response. Obtain blood gases when clinically indicated, particularly when hypercapnia is possible.

Preoxygenation before intubation

Preoxygenation increases the oxygen available in the lungs before a planned period of apnea, helping delay desaturation during intubation. When feasible, position the patient head-up, apply a well-sealed mask with high-concentration oxygen, and allow adequate time for effective breathing. Poor mask seal, agitation, severe lung disease, and shunt physiology can make preoxygenation less effective.

Choose the method to match the patient:

• A face mask is appropriate for many patients when a good seal can be maintained.

• Consider high-flow nasal oxygen when laryngoscopy is expected to be difficult; it can often remain in place during attempts.

• For severe hypoxemia, SCCM suggests preoxygenation with noninvasive ventilation rather than relying on high-flow nasal oxygen alone.

• If agitation or delirium prevents a patient from tolerating a mask or another preoxygenation method, an appropriately trained team may consider medication-assisted preoxygenation.

Preoxygenation reduces, but does not eliminate, desaturation risk. Continue monitoring and have a plan to restore oxygenation if an attempt is prolonged or unsuccessful.

Responding to hypoxemia

Treat hypoxemia as an urgent sign. Address oxygen delivery and the cause, and do not delay treatment in a critically ill patient while troubleshooting a monitor.

1. Assess the patient and verify the reading. Check the pulse-oximeter signal, probe placement, and perfusion. Look for cyanosis, altered mental status, increased work of breathing, and exhaustion.

2. Support the airway and position. Reposition the head and torso, open or suction the airway when indicated, and correct an obvious obstruction or poor mask seal.

3. Increase support and reassess. Choose an appropriate higher-support oxygen device or ventilatory support, then promptly reassess saturation and clinical response. With a reservoir mask, ensure adequate flow and an inflated reservoir. With bag-mask ventilation, focus on an effective seal and chest rise.

4. Look for the cause. Consider airway obstruction, bronchospasm, aspiration, pneumonia, pulmonary edema, pneumothorax, or worsening respiratory failure. Oxygen alone may not resolve the underlying problem.

5. Escalate early if the patient is deteriorating. Persistent hypoxemia, worsening work of breathing, declining consciousness, or inadequate ventilation warrants urgent senior or airway-team involvement and consideration of advanced respiratory support. Do not let repeated device changes delay definitive care.

For a patient at risk of hypercapnic respiratory failure, do not withhold oxygen during a life-threatening emergency. Provide the oxygen needed to stabilize the patient, then titrate to an appropriate target and obtain clinical reassessment and blood-gas testing as indicated.